This invention relates to apparatus for preventing complete discharge of a battery, and more particularly, to an apparatus for preventing complete discharge of a battery used in a powered vehicle such as an electric automobile.
In recent years pollution caused by automobile exhaust gases has come to be viewed as a serious problem. For this reason, the electric automobile is in the limelight for implementation as a non-polluting automobile. This electric automobile commonly uses storage cells such as Ni--Cd (nickel-cadmium) batteries as its power source. Specifically, with the Ni--Cd battery used as the power source, it is repeatedly recharged and reused whenever battery voltage drops off. However, continuous discharge of the Ni--Cd battery eventually results in complete discharge, causing drastic reduction in the lifetime of the battery. Consequently, to keep the battery from discharging completely, a means of detecting depleted battery voltage and disconnecting the battery from the driving circuit of the electric automobile is required.
In general, the motion of an electric automobile is complex, and the state of the storage battery corresponding to that motion is also complex. The storage battery's terminal voltage as well as voltage level discharge current varies over a wide range. For example, when a large load is put on the electric automobile during sudden acceleration from a stop or in overcoming a steep hill, very large discharge currents flow momentarily from the storage battery and the terminal voltage exhibits a corresponding in voltage level large change. When the automobile returns to steady state motion, the discharge current from the storage battery decreases, and the change in terminal voltage consequently settles down. In this fashion, storage battery parameters describe a complex pattern corresponding to the motion of the electric automobile.
Formerly, it has been common practice to measure storage battery terminal voltage and disconnect the battery from the driving circuit at a prescribed voltage. The problem with this is that, as described above, since storage battery terminal voltage varies wildly corresponding to electric automobile motion, terminal voltage can drop and the storage battery can be disconnected from the driving circuit even though the battery still has sufficient reserve power capacity. Consequently, this results in very short electric automobile driving time. Considering this situation and setting the prescribed voltage lower can result in steady state operation of the electric automobile without terminal voltage reduction below the prescribed voltage, continued battery discharge, and finally complete battery discharge. To solve the above mentioned problems, the present invention responds to the motion of an electric automobile to accurately prevent complete storage battery discharge. Specifically, the storage battery is disconnected from the driving circuit of the electric automobile when storage battery parameters corresponding to automobile motion satisfy prescribed conditions.